Help me understand pregnancy week calculations please?

Sorry if this sounds totally stupid but can someone explain how pregnancy weeks are measured?

I believe I ovulated on 17th June. We DTD on 16th, 18th and 19th. I got a positive test on 28th.

I have been using the Ovia pregnancy app since, and it notes today as the start of my 4th week of pregnancy. However I know I conceived eith 13, 15 or 16 days ago, which surely makes me more like 2 weeks pregnant?

I have done some googling and it seems pregnancy it measured from the start of your previous period just cause that's easier to measure. My query now though, is will I have the 12 week scan at the real 12 weeks since conception, or the 12 weeks of pregnancy (when I'd actually be more like 10 weeks)? I'm trying to plan when I can tell people and also when to tell work (complicated contract issue) so this detail is actually quite important... and I am very confused!

Hi. Everything is counted from the date of your last period. So your scan will be around 12 weeks of the start of your last period, your due date will be 40 weeks from the start of your last period (until your 12 week scan where it will be set based on size chart). Your actual conception date isn't really relevant for anything. Many people won't actually know their real conception (or even ovulation date).

You are generally seen as time since conception plus 2 weeks. So, as you've found, 2 weeks after conception you are 4 weeks pregnant. 12 weeks scan would be roughly 10 weeks after conception. It's just done this way as historically people wouldn't know when they'd conceived, so counting based on an average menstrual cycle was more straightforward.